What do mosses tell us about how phytochrome works?

What do mosses tell us about how phytochrome works?
Jon Hughes, Mathias Zeidler & Franz Mittmann
Plant Physiology, Justus Liebig University, D35390 Giessen, Germany
e-mail: [email protected]; phone: +496419935430; fax +496419935429
The red / far-red photochromic photoreceptor phytochrome plays a critical role in
steering development throughout the lifecycle of plants. However, its mechanism of
action and signal transduction is unclear, both classical signal-transduction
intermediates such as cGMP, Ca2+/CAM and G proteins, as well as a much more
specific signaling system involving Pr/Pfr state-dependent nuclear transport and direct
activation of transcription factors such as PIF3 having been implicated. The
phytochrome-dependent polaro- and phototropic behavior of moss protonemal tip cells is
rather interesting in this context. These responses require not only that local Pfr levels
within the cytoplasm be sensitive to (1) the direction and (2) the polarization of
irradiation, but also (3) that this vectorial information be retained by the signal
transduction system which coordinates the direction of cell growth. Explanations of (1)
and (2) are incompatible with conventional wisdom that phytochrome is dissolved freely
and distributed anisotropically in the cytoplasm, while (3) is clearly incompatible with an
action mechanism involving transcriptional control alone. These three paradoxes
indicate that we are missing and/or misunderstanding fundamental aspects of the
phytochrome system in mosses, a fact that would be of precious little significance were
the same not also true for the phytochrome system in higher plants.
Homologous recombination in Physcomitrella1 (and Ceratodon2) together with the
genome sequence opens unique possibilities for studying plant gene function using
moss systems. While it is likely that some aspects phytochrome action in mosses are
peculiar to that group, it is rather unlikely that at the mechanism of phytochrome action is
fundamentally different in mosses and higher plants. Thus we expect that studies in
mosses will provide insights into phytochrome biology in general.
Prior to the genome sequencing project we cloned all four PHY genes from
Physcomitrella (PP1-PP4)3. Phylogenetically, these represent two distinct clades with
the two canonical phytochromes from Ceratodon (CP2 and CP3). Fluence rate /
response curves of the wildtype revealed complex for phototropic behavior with both
positive and negative (avoidance) regions. The response is strictly red-light-dependent
and is far-red reversible, indicating phytochrome involvement. Each Physcomitrella
phytochrome was successfully targeted, mutant lines showing a lesion in one aspect of
the phototropic response. Searches of the nascent Physcomitrella genome sequence
reveal that many but not all of the putative higher plant phytochrome signal transduction
components are represented. We plan to study knockouts of these and of novel candidates
revealed by two-hybrid methods in respect of their photo- and cell physiology.
1. Schaefer,D.G. & Zrÿd,J.-P. Efficient gene targeting in the moss Physcomitrella patens. Plant J. 11,
1195-1206 (1997).
2. Brücker,G., Mittmann,F., Hartmann,E. & Lamparter,T. Targeted site-directed mutagenesis of a heme
oxygenase locus by gene replacement in the moss Ceratodon purpureus. Planta 220, 864-874 (2005).
3. Mittmann,F. et al. Targeted knockout in Physcomitrella reveals direct actions of phytochrome in the
cytoplasm. Proc. Natl. Acad. Sci. U. S. A 101, 13939-13944 (2004).